Method for producing an aqueous polymer dispersion from vinylaromatic compound and conjugated aliphatic diene

ABSTRACT

The present invention relates to a process for producing an aqueous polymer dispersion by free-radically initiated aqueous emulsion polymerization, by polymerizing, in an aqueous medium,(a) 40 to 75 parts by weight of at least one vinylaromatic compound and(b) 24.9 to 59.9 parts by weight of at least one conjugated aliphatic diene(c) 0.1 to 10 parts by weight of at least one monomer comprising acid groups and(d) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer,the amounts of the monomers (a) to (d) adding up to 100 parts by weight, in a monomer feed process in the presence of a free-radical initiator, with the proviso thatthe vinylaromatic compound is metered in under polymerization conditions in a continuous mass flow over a period of at least 120 minutes,the metering of the conjugated aliphatic diene is started at a time at which at least 5% and not more than 30% of the vinylaromatic compound has already been metered in in a continuous mass flow andoptionally, the polymerization is initiated in an aqueous polymerization mixture comprising up to 5% of the vinylaromatic compound and comprising no aliphatic diene,and also to the aqueous polymer dispersions produced by the process and to the use of these as a binder, adhesive, sizing agent for fibers, for the production of coatings or for the production of a paper coating slip.

The invention relates to a process for producing an aqueous polymer dispersion, wherein a vinylaromatic compound and a conjugated aliphatic diene are copolymerized in aqueous medium in the presence of a free-radical initiator. The invention also relates to the aqueous polymer dispersions produced by the process and to the use of these as a binder, adhesive, sizing agent for fibers, for the production of coatings or for the production of a paper coating slip.

Known binders for paper coating slips based on copolymers of vinylaromatic compounds and aliphatic dienes are not yet completely satisfactory in all respects. In the case of aqueous emulsion copolymers formed from vinylaromatic compounds and aliphatic dienes, side reactions of the dienes lead to the formation of intensely odorous Diels-Alder adducts.

EP 2 370 484 teaches a process for producing styrene-butadiene/acrylonitrile dispersions comprising a low level of coagulate, in which the metering of the acrylonitrile is not begun with the start of the polymerization but rather in a delayed manner after a third of the feed time of the remaining monomers has passed.

WO 99/09251 discloses a process for producing styrene-butadiene copolymers in which the polymerization is carried out such that firstly a portion of the monomer mixture and of the free-radical initiator is added to the aqueous solution or dispersion of degraded starch which has been heated to polymerization temperature, and the remaining monomer mixture is metered in continuously after the start of the polymerization.

In addition, EP 1 408 059 teaches a process for producing a styrene-butadiene latex by metering in styrene and butadiene in each case along a discontinuous gradient, in that the addition amount of the butadiene is constantly reduced and the addition amount of the styrene is constantly increased. The polymerization is started using ammonium persulfate as free-radical initiator, the metering of all monomers starting at the same time. In the course of this, the addition is continued, being increased or decreased multiple times discontinuously. However, dispersions obtained in this way have only slightly reduced residual amounts of 4-phenylcyclohexene (4-PCH) compared to the process using a continuous gradient. However, it is common to all of these processes using discontinuous addition of the monomers that sulfur regulators are required.

It is an object of the invention to provide a process for producing aqueous polymer dispersions based on copolymers formed from vinylaromatics and conjugated aliphatic dienes which have a lower proportion of 4-phenylcyclohexene. In addition, when used in paper coating slips they should display good performance properties, in particular cause a low degree of yellowing of the paper.

The object is achieved according to the invention by a process for producing an aqueous polymer dispersion by free-radically initiated aqueous emulsion polymerization, by polymerizing, in an aqueous medium,

(a) 40 to 75 parts by weight of at least one vinylaromatic compound and (b) 24.9 to 59.9 parts by weight of at least one conjugated aliphatic diene (c) 0.1 to 10 parts by weight of at least one monomer comprising acid groups and (d) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, the amounts of the monomers (a) to (d) adding up to 100 parts by weight, in a monomer feed process in the presence of a free-radical initiator, with the proviso that

-   -   the vinylaromatic compound is metered in under polymerization         conditions in a continuous mass flow over a period of at least         120 minutes,     -   the metering of the conjugated aliphatic diene is started at a         time at which at least 5% and not more than 30% of the         vinylaromatic compound has already been metered in in a         continuous mass flow and     -   optionally, the polymerization is initiated in an aqueous         polymerization mixture comprising up to 5% of the vinylaromatic         compound and comprising no aliphatic diene.

When reference is made in the context of this application to 5% of the vinylaromatic compound, this is based on the total amount of vinylaromatic compound.

If an amount is reported in parts by weight hereinafter, this is based on 100 parts by weight of total monomers, unless specified otherwise.

The following ethylenically unsaturated monomers (a), (b), (c) and (d) can be used to produce the aqueous polymer dispersions.

Examples of suitable vinylaromatic compounds (monomers of group (a)) include styrene, α-methylstyrene and/or vinyltoluene. From this group of monomers, preference is given to choosing styrene.

The total amount of monomers (a) is 40 to 75 parts by weight and preferably 45 to 70 parts by weight, in particular 50 to 65 parts by weight, based on 100 parts by weight of total monomers (a to d).

Examples of conjugated aliphatic diene (monomers of group (b)) which may be mentioned include 1,3-butadiene, isoprene, 1,3-pentadiene, dimethyl-1,3-butadiene and cyclopentadiene. From this group of monomers, preference is given to using 1,3-butadiene and/or isoprene.

The total amount of monomers (b) is 24.9 to 59.9 parts by weight, preferably 29.9 to 54.9 parts by weight and in particular 34.9 to 49.9 parts by weight, based on 100 parts by weight of total monomers.

Examples of monomers comprising acid groups (monomers (c)) which may be mentioned include ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids and vinylphosphonic acid. The ethylenically unsaturated carboxylic acids used are preferably α,β-monoethylenically unsaturated mono- and dicarboxylic acids having 3 to 6 carbon atoms in the molecule. Examples of these are acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid and vinyllactic acid. Examples of suitable ethylenically unsaturated sulfonic acids include vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate and sulfopropyl methacrylate. Preference is given to using acrylic acid, methacrylic acid and itaconic acid. The cited acids may be used either as a single component or as a combination thereof.

The monomers comprising acid groups may be used in the polymerization in the form of the free acids or else in a form partially or completely neutralized by suitable bases. Preference is given to using sodium hydroxide solution, potassium hydroxide solution or ammonia as neutralizing agent.

The total amount of monomers (c) is 0.1 to 10 parts by weight, preferably 0.1 to 8 parts by weight or 1 to 6 parts by weight of one or more monomers comprising acid groups, based on 100 parts by weight of total monomers.

Other monomethylenically unsaturated monomers (d) are monomers which differ from the monomers of groups (a), (b), and (c). They are preferably selected from acrylamide and methacrylamide, vinyl esters of saturated C₁ to C₁₈ carboxylic acids, preferably vinyl acetate, and esters of acrylic acid and methacrylic acid with monohydric C₁ to C₁₈ alcohols such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, pentyl acrylates, pentyl methacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, allyl esters of saturated carboxylic acids, vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturated carboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates, vinyl chloride and vinylidene chloride (monomers of group (d)).

This group of monomers is optionally used for modification of the polymers. The total amount of all other monomers may be up to 20 parts by weight based on 100 parts of total monomer. Based on 100 parts by weight of the total monomers, the proportion of one or more monomers of group (d) is 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight and in particular 0.5 to 10 parts by weight.

If this comprises acrylonitrile and/or methacrylonitrile, the amount thereof used is preferably 2 to 12 parts by weight and in particular 4 to 9 parts by weight, based on 100 parts by weight of total monomer.

Preference is given to a process in which the vinylaromatic compound is styrene and/or methylstyrene and the conjugated aliphatic diene is 1,3-butadiene and/or isoprene. The process is particularly advantageous for producing styrene-butadiene dispersions.

It is advantageous to polymerize

(a) 60 to 75 parts by weight of at least one vinylaromatic compound and (b) 24.9 to 39.9 parts by weight of at least one conjugated aliphatic diene (c) 0.1 to 8 parts by weight of at least one monomer comprising acid groups and (d) 0 to 10 parts by weight of at least one other monoethylenically unsaturated monomer, the amounts of the monomers (a) to (d) adding up to 100 parts by weight.

Particular preference is given to polymerizing

(a) 60 to 70 parts by weight of at least one vinylaromatic compound (b) 29 to 39 parts by weight of at least one conjugated aliphatic diene (c) 1 to 6 parts by weight of at least one monomer comprising acid groups and (d) 0 to 5 parts by weight of at least one other monoethylenically unsaturated monomer, the amounts of the monomers (a) to (d) adding up to 100 parts by weight.

The emulsion polymerization is effected in an aqueous medium. This can for example be fully deionized water or else mixtures of water and a solvent miscible therewith such as methanol, ethanol, ethylene glycol, glycerol, sugar alcohols such as sorbitol or tetrahydrofuran. The total amount of aqueous medium is proportioned here such that the aqueous polymer dispersion obtained has a solids content of 20% to 70% by weight, frequently 30% to 65% by weight and often 40% to 60% by weight.

The process according to the invention uses free-radical initiators (also referred to as free-radical polymerization initiators), that is to say initiators which form free radicals under the reaction conditions. These may be peroxides or they may be azo compounds. Redox initiator systems are of course also suitable.

Peroxides used may in principle be inorganic peroxides and/or organic peroxides. Examples of suitable inorganic peroxides include hydrogen peroxide and peroxodisulfates, such as the mono- or dialkali metal or ammonium salts of peroxodisulfuric acid, for example the mono- and disodium, mono- and dipotassium, or ammonium salts thereof. Examples of suitable organic peroxides are alkyl hydroperoxides such as tert-butyl hydroperoxide, aryl hydroperoxides such as p-menthyl or cumene hydroperoxide, and dialkyl or diaryl peroxides such as di-tert-butyl, dibenzoyl or dicumene peroxide.

Azo compounds used are essentially 2,2′-azobis(isobutyronitrile), 2,2′-azobis(2-methylbutyronitrite), 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(N,N′-dimethyleneisobutyroamidine) dihydrochloride and 2,2′-azobis(amidinopropyl) dihydrochloride (AIBA, corresponding to V-50 from Wako Chemicals).

Redox initiator systems are combined systems made up of at least one organic or inorganic reducing agent and at least one peroxide. Suitable oxidants for redox initiator systems are essentially the peroxides mentioned above. Corresponding reducing agents that may be used are sulfur compounds in a low oxidation state such as alkali metal sulfites, for example potassium and/or sodium sulfite, alkali metal hydrogen sulfites, for example potassium and/or sodium hydrogen sulfite, alkali metal metabisulfites, for example potassium and/or sodium metabisulfite, acetone bisulfite, formaldehyde sulfoxylates, for example potassium and/or sodium formaldehyde sulfoxylate, alkali metal salts, specifically potassium and/or sodium salts of aliphatic sulfinic acids and alkali metal hydrogen sulfides, for example potassium and/or sodium hydrogen sulfide, salts of polyvalent metals, such as iron(II) sulfate, iron(II) ammonium sulfate, iron(II) phosphate, enediols, such as dihydroxymaleic acid, benzoin and/or ascorbic acid, and reducing saccharides, such as sorbose, glucose, fructose and/or dihydroxyacetone.

Preferred free-radical initiators are inorganic and organic peroxides, preferably ammonium or alkali metal salts of peroxosulfates or peroxodisulfates, and tert-butyl, p-menthyl and cumyl hydroperoxide, in particular selected from sodium and potassium peroxodisulfate, tert-butyl hydroperoxide and cumyl hydroperoxide. Particular preference is given here to using both at least one inorganic peroxide, preferably peroxodisulfate, in particular sodium peroxodisulfate, and one organic peroxide, preferably alkyl hydroperoxide, in particular t-butyl hydroperoxide.

The polymerization is generally effected using 0.1 to 5 parts by weight of the free-radical initiator, preferably 0.5 to 4 parts by weight of the free-radical initiator, preferably at least one inorganic and/or organic peroxide, based in each case on 100 parts by weight of total monomers. The ratio of the total amount of inorganic peroxide to the total amount of organic peroxide is preferably 1/10 to 10/1, preferably 1/5 to 5/1, in particular 1/3 to 3/1, viewed over the whole process.

Initiation of the polymerization reaction is understood to mean the start of the polymerization reaction of the monomers present in the polymerization vessel as a result of decomposition of the free-radical initiator.

In order to polymerize the monomers, firstly an aqueous solution is prepared comprising a protective colloid and/or an emulsifier in dissolved form and optionally polystyrene seed. This mixture is preferably heated to the temperature at which the monomers are intended to be polymerized. As soon as the respectively desired polymerization temperature has been reached or within a time period of 1 to 15 minutes, preferably 1 to 10 minutes, after reaching the polymerization temperature, metering of the monomers (a) and (c) and optionally (d) is started. According to this variant, no monomers are initially charged. The polymerization starts when the polymerization mixture comprises vinylaromatic compound and free-radical initiator and reaches a temperature in the range from 80° C. to 95° C.

According to a preferred embodiment, the polymerization is initiated in an aqueous polymerization mixture comprising up to 5% of the vinylaromatic compound and no aliphatic diene. According to this variant, monomers are initially charged. The polymerization then starts when

-   -   0.1% to 5% of the vinylaromatic compound has been initially         charged,     -   the reaction temperature has been set to a temperature in the         range from ≥80° C. to ≤95° C. and     -   free-radical initiator, preferably 0.1 to 0.5 parts by weight of         an inorganic peroxide based on 100 parts by weight of total         monomer, has been initially charged.

The order in which these conditions are established is not crucial. The vinylaromatic compound is preferably initially charged and the inorganic peroxide added after heating to the reaction temperature in the range from 80−95° C., initiating the polymerization.

According to a preferred variant of the process, a proportion of inorganic peroxide, preferably an ammonium salt or an alkali metal salt of a peroxodisulfate, is selected for initiating the polymerization and then the remaining free-radical initiator amount, preferably the inorganic peroxide and an organic peroxide, is metered in in a continuous mass flow. Here, preference is given to starting the metering of the organic peroxide starting from a time at which at least 5% and not more than 20% of the vinylaromatic compound has already been metered in in a continuous mass flow.

Particular preference is given to selecting 0.1 to 0.5 part by weight of inorganic peroxide, preferably an ammonium salt or an alkali metal salt of a peroxodisulfate, for initiating the polymerization and then directly metering in 0.1 to 2.0 parts by weight of inorganic peroxide, and starting the metering of 0.1 to 2.0 parts by weight, based in each case on 100 parts by weight of total monomer, of the organic peroxide starting from a time at which at least 5% and not more than 20% of the vinylaromatic compound has already been metered in, wherein the metering of the inorganic peroxide, the organic peroxide and also the vinylaromatic compound is effected in each case in a continuous mass flow.

According to a preferred variant, the inorganic peroxide is continuously metered in throughout the polymerization following the initiation.

Preference is also given to a process in which an organic peroxide is metered in starting from a time at which at least 5%, particularly preferably at least 8, in particular at least 10%, and not more than 20% of the total amount of vinylaromatic compound has already been metered in under polymerization conditions.

Polymerization conditions are generally understood to mean those amounts of free-radical initiator, temperatures and pressures under which the free-radically initiated aqueous emulsion polymerization does not come to a halt. The polymerization depends on the nature and amount of the free-radical initiator used.

Preferred polymerization conditions are a temperature in the range from ≥80° C. to ≤105° C., preferably ≥85° C. to ≤100° C. The metering of the conjugated aliphatic diene is generally effected at elevated pressure. The metering of the conjugated aliphatic diene preferably takes place at a pressure in the range from 5 to 15 bar. The elevated pressure has the effect that for example the 1,3-butadiene which is gaseous at standard pressure and room temperature largely resides in the polymerization mixture.

The monomers are preferably metered in continuously, that is say without interruption. In this case, the monomers are preferably metered in with a metering rate which deviates from the average value of the respective overall feed by no more than 30%, preferably by no more than 20%. According to a preferred embodiment, the metering rate of the monomers (increase in the monomers) corresponds approximately to the polymerization rate of the monomers (decrease in the monomers).

According to the process according to the invention, the conjugated aliphatic diene is only metered in starting from a time at which at least 5%, preferably at least 8, in particular at least 10%, of the total amount of vinylaromatic compound has already been metered in in a continuous mass flow.

The vinylaromatic compound is metered in under polymerization conditions preferably in a continuous mass flow over a period of at least 120 minutes, preferably over a period of 180 to 300 minutes, especially over a period of 210 to 270 minutes.

The conjugated aliphatic diene is metered in in a continuous mass flow preferably over a period of at least 60 minutes, particularly preferably over a period of 120 to 240 minutes, especially over a period of 150 to 210 minutes, wherein particularly preferably the vinylaromatic compound is additionally metered in in a continuous mass flow over a period of at least 120 minutes, preferably over a period of 180 to 300 minutes, especially over a period of 210 to 270 minutes.

According to one embodiment of the invention, the polymerization is conducted in the presence of a degraded starch. Preference is given in the emulsion copolymerization to using 15 to 100 parts by weight of a degraded starch per 100 parts by weight of the monomers.

Suitable starting starches for producing the degraded starches to be used according to the invention are all native starches such as starches from corn, wheat, oats, barley, rice, millet, potatoes, peas, tapioca, sorghum or sago. Starting starches of interest are in addition those natural starches having a high amylopectin content such as waxy corn starch and waxy potato starch. The amylopectin content of these starches is above 90%, usually 95 to 100%.

In addition, starches which have been modified by etherification or esterification may be used to produce the polymer dispersions according to the invention. The modification usually also results in degradation. Such products are known and are commercially available. They are produced for example by esterification of native starches or degraded native starches with inorganic or organic acids, their anhydrides or chlorides. Phosphated and acetylated degraded starches are of particular interest. The most commonly used methods for the etherification of starches consists in treating starch with organic halogen compounds, epoxides or sulfates in aqueous alkaline solution. Known starch ethers are alkyl ethers, hydroxyalkyl ethers, carboxyalkyl ethers and allyl ethers. The reaction products of starches with 2,3-epoxypropyltrimethylammonium chloride are also suitable.

Further suitable starches are cationically modified starches, that is to say starch compounds having amino groups or ammonium groups.

The starches can be degraded enzymatically, oxidatively or hydrolytically by the action of acids or bases. The degradation of starch is generally known and is described in EP 2580257, for example. Degraded starches are commercially available. A degraded starch can be introduced for the polymerization or one can be prepared in situ and the polymerization then conducted in the presence thereof.

Particular preference is given to degraded native starches, in particular native starches degraded to maltodextrin.

Preference is given to degraded starches having an intrinsic viscosity ηi of ≤0.07 dl/g or ≤0.05 dl/g. The intrinsic viscosity ηi of the degraded starches is preferably in the range from 0.02 to 0.06 dl/g. The intrinsic viscosity ηi is determined in accordance with DIN EN 1628 at a temperature of 23° C.

In order to promote the emulsification of the monomers in the aqueous medium, it is possible to use the typically used protective colloids and/or emulsifiers. An extensive description of suitable protective colloids can be found in Houben-Weyl, Methoden der organischen Chemie [Methods of Organic Chemistry], volume XIV/1, Makromolekulare Stoffe [Macromolecular Materials], Georg-Thieme-Verlag, Stuttgart, 1961, pages 411 to 420.

Useful emulsifiers include interface-active substances having a number-average molecular weight of typically below 2000 g/mol or preferably below 1500 g/mol, whereas the number-average molecular weight of the protective colloids is above 2000 g/mol, for example from 2000 to 100 000 g/mol, in particular from 5000 to 50 000 g/mol.

Examples of suitable emulsifiers are ethoxylated C₈ to C₃₆ fatty alcohols having a degree of ethoxylation of 3 to 50, ethoxylated mono-, di- and tri-C₄- to -C₁₂-alkylphenols having a degree of ethoxylation of 3 to 50, alkali metal salts of dialkyl esters of sulfosuccinic acid, alkali metal and ammonium salts of C₈- to C₁₂-alkyl sulfates, alkali metal and ammonium salts of C₁₂- to C₁₈-alkylsulfonic acids and alkali metal and ammonium salts of C₉- to C₁₈-alkylarylsulfonic acids. Cationic emulsifiers are for example compounds having at least one amino or ammonium group and at least one C₈-C₂₂-alkyl group.

Further suitable emulsifiers have been found to be compounds of the general formula I

in which R¹ and R² are C₄- to C₂₄-alkyl and one of the R¹ or R² radicals may also be hydrogen, and A and B may be alkali metal ions and/or ammonium ions. In the general formula I, R¹ and R² are preferably linear or branched alkyl radicals having 6 to 18 carbon atoms, especially having 6, 12 or 16 carbon atoms, or hydrogen atoms, where R¹ and R² are not both simultaneously hydrogen atoms. A and B are preferably sodium, potassium or ammonium ions, with sodium ions being particularly preferred. Particularly advantageous compounds I are those in which A and B are sodium ions, R¹ is a branched alkyl radical having 12 carbon atoms and R² is a hydrogen atom or R¹. Technical grade mixtures comprising a proportion of 50% to 90% by weight of the monoalkylated product, for example Dowfax® 2A1 (brand of Dow Chemical Company), are frequently used. The compounds I are common knowledge, for example from U.S. Pat. No. 4,269,749, and are commercially available.

If emulsifiers and/or protective colloids are additionally used as auxiliaries for dispersing the monomers, the amounts used thereof are for example 0.1 to 5 parts by weight based on 100 parts by weight of monomers.

According to one variant of the process, the polymerization is conducted in the presence of a polystyrene seed, preferably in the presence of an aqueous dispersion of finely divided polystyrene having an average particle diameter of 20 to 40 nm (determined by ultracentrifugation).

In order to modify the properties of the polymers, it is possible to conduct the emulsion polymerization optionally in the presence of at least one chain transfer agent which does not comprise sulfur or halogen. These are typically used to reduce or to control the molecular weight of the polymers obtainable by a free-radical aqueous emulsion polymerization.

The process according to the invention preferably should not comprise the use of chain transfer agent selected from aliphatic and/or araliphatic halogen compounds, organic thio compounds and substituted thiols during the polymerization.

Examples of chain transfer agents which do not comprise sulfur or halogen are aldehydes such as formaldehyde, acetaldehyde and propionaldehyde, organic acids such as formic acid, sodium formate or ammonium formate, alcohols such as in particular isopropanol, and phosphorus compounds such as sodium hypophosphite. If during the polymerization a chain transfer agent is used, the respective amount used is for example 0.01 to 5, preferably 0.1 to 1 parts by weight, based on 100 parts by weight of the monomers used during the polymerization. The chain transfer agents are preferably metered together with the monomers into the initial charge. However, they may also be present in part or in full in the initial charge. They can also be metered in stepwise in a staggered manner with respect to the monomers. Particular preference is given to a process in which no chain transfer agent is present during the polymerization.

To complete the polymerization reaction, it is sufficient in most cases to stir the reaction mixture after all monomers have been added for example for a further 1 to 3 hours at the polymerization temperature. A conversion of around 95% has typically been achieved at this point in time.

In order to increase the conversion yet further, it is possible for example to add further free-radical initiator from the group of the abovementioned initiators to the reaction mixture or to prolong the addition and what is known as a “postpolymerization”, that is to say a polymerization to achieve conversions of >95% up to 99%.

Such a postpolymerization can be conducted at the same, lower or else at higher temperature as/than the main polymerization. For example, 0.1 to 1.5 parts by weight, based on 100 parts by weight of the monomers used in the polymerization, of inorganic peroxide, preferably sodium peroxodisulfate, are metered in in this phase as initiator and the polymerization temperature is set to a temperature in the range from 80 to 120° C.

The pH can be for example 1 to 5 during the polymerization. After the end of the polymerization at a conversion of >95%, the pH is for example adjusted to a value between 6 and 7.

Chemical deodorization can in addition also be performed. If traces of residual monomers are still to be removed, this can also be done chemically by the action of redox initiator systems, as specified in DE-A 44 35 423, DE-A 44 19 518 and in DE-A 44 35 422. Suitable oxidizing agents are in particular the abovementioned organic and/or inorganic peroxides. Suitable reducing agents preferably include sodium disulfite, sodium hydrogen sulfite, sodium dithionite, sodium hydroxymethanesulfinate, formamidinesulfinic acid, acetone bisulfite (=addition product of sodium hydrogensulfite onto acetone), ascorbic acid or reducing sugar compounds, or water-soluble mercaptans such as mercaptoethanol.

The treatment with the redox initiator system is conducted in a temperature range from 60 to 100° C., preferably at 70 to 90° C. The redox partners can each independently be added to the dispersion in their entirety, in portions or continuously over a period of 10 minutes to 4 hours. To improve the postpolymerization action of the redox initiator systems, soluble salts of metals of varying valency, such as iron, copper or vanadium salts, may be added to the dispersion. Complexing agents which keep the metal salts in solution under the reaction conditions are also frequently added.

Following the polymerization reaction (main polymerization+postpolymerization) and optional chemical deodorization, it may be necessary to render the aqueous polymer dispersions largely free from odor carriers such as residual monomers and other volatile organic constituents, which is also referred to as physical deodorization. This can be achieved in a manner known per se by physical means by distillative removal (in particular via steam distillation) or by stripping with an inert gas.

The present invention also relates to the dispersions obtainable by the process according to the invention. These are virtually coagulate-free aqueous dispersions. The amount of coagulate is in the ppm range and is preferably less than 100 ppm, in particular less than 50 ppm. They additionally have a uniform particle size and in general have a solids content of approx. 50% by weight, preferably in the range from 45% to 55% by weight. The dispersions according to the invention have a low proportion of 4-phenylcyclohexene.

The aqueous polymer dispersions according to the invention are used as a binder, adhesive, sizing agent for fibers, for the production of coatings or for the production of paper coating slips. The aqueous polymer dispersions according to the invention are suitable both for the sizing of textile fibers and for the sizing of mineral fibers, especially glass fibers. Because of their good adhesive strength, in particular when using comonomers which lead to a low glass transition temperature of the copolymer (e.g. less than 20° C.), they can also be used as an adhesive for example for the production of laminates and for the production of coatings such as for example barrier coatings. The aqueous polymer dispersions according to the invention are preferably used as binders in paper coating slips.

The invention accordingly also provides a paper coating slip, comprising

-   (i) inorganic pigment and -   (ii) an aqueous polymer dispersion described above and obtainable by     the process according to the invention -   (iii) and optionally further auxiliaries.

Besides water, paper coating slips generally comprise pigments, binders and auxiliaries for establishing the required rheological properties, for example thickeners. The pigments are typically dispersed in water. The paper coating slip comprises pigments in an amount of preferably at least 80% by weight, for example 80% to 95% by weight or 80% to 90% by weight, based on the total solids content.

White pigments are especially suitable. Examples of suitable pigments are metal salt pigments such as for example calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate and calcium carbonate, among which carbonate pigments and especially calcium carbonate are preferred. The calcium carbonate can be ground calcium carbonate (GCC, natural ground calcium carbonate), precipitated calcium carbonate (PCC), lime or chalk. Suitable calcium carbonate pigments are available for example as Covercarb® 60, Hydrocarb® 60 or Hydrocarb® 90 ME. Further suitable pigments are for example silicas, aluminum oxides, aluminum hydroxide, silicates, titanium dioxide, zinc oxide, kaolin, alumina, talc or silicon dioxide. Suitable further pigments are available for example as Capim® MP 50 (clay), Hydragloss® 90 (Clay) or Talcum C10.

The paper coating slip comprises the polymer dispersion produced according to the invention as the sole binder or in combination with further binder. The most important functions of binders in paper coating slips are to bind the pigments to the paper and the pigments to each other and to some extent to fill cavities between pigment particles.

For example, 1 to 50 parts by weight, preferably 1 to 25 parts by weight or 5 to 20 parts by weight of the polymer according to the invention are used (in terms of solids, i.e. without water or other solvents which are liquid at 21° C. and 1 bar) per 100 parts by weight of pigments.

Preference is given to a paper coating slip which comprises the polymers of the aqueous polymer dispersion in an amount of 1 to 50 parts by weight based on the total amount of pigments, and pigments in an amount of 80 to 95 parts by weight based on the total solids content, and an auxiliary, and the pigments of which are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminum oxides, aluminum hydroxide, silicates, titanium dioxide, zinc oxide, kaolin, alumina, talc and silicon dioxide, and the auxiliary of which is selected from the group consisting of thickeners, further polymeric binders, co-binders, optical brighteners, fillers, leveling agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.

The further synthetic binders which differ from the polymers produced according to the invention are generally known and are described for example in D. Urban and K. Takamura, Polymer Dispersions and Their Industrial Applications, 2002, Wiley-VCH Verlag GmbH, Weinheim, chapter 4.4.4, page 90 ff., the disclosure of which is expressly incorporated by reference.

Useful further binders include binders with a natural basis, in particular starch-based binders, and synthetic binders which differ from the polymers produced according to the invention, especially emulsion polymers which can be produced by emulsion polymerization. In this context, “starch-based binders” is to be understood as meaning any native, modified or degraded starch. Native starches can consist of amylose, amylopectin or mixtures thereof. Modified starches can be oxidized starches, starch esters or starch ethers. The molar mass of the starch can be reduced by hydrolysis (degraded starch). Oligosaccharides or dextrins are possible degradation products. Preferred starches are cereal, corn and potato starches. Particular preference is given to cereal starch and corn starch, very particular preference is given to corn starch.

Paper coating slips according to the invention may additionally comprise further auxiliaries, for example fillers, co-binders and thickeners for further optimizing viscosity and water retention, optical brighteners, dispersants, surfactants, lubricants (e.g. calcium stearate and waxes), neutralizing agents (e.g. NaOH or ammonium hydroxide) for adjusting pH, defoamers, deaerators, preservatives (e.g. biocides), leveling agents, dyes (in particular soluble dyes) etc. Useful thickeners include not only synthetic polymers (e.g. crosslinked polyacrylate), but also in particular celluloses, preferably carboxymethyl cellulose. Optical brighteners are for example fluorescent or phosphorescent dyes, in particular stilbenes.

An aqueous paper coating slip is preferably involved; this comprises water in particular already as a result of the preparation form of the constituents (aqueous polymer dispersions, aqueous pigment slurries); the desired viscosity can be set via addition of further water. Customary solids contents of the paper coating slips are in the range from 30% to 80% by weight. The pH of the paper coating slip is preferably set to values of 6 to 11, in particular 7 to 10.

The invention also provides paper or board coated with a paper coating slip according to the invention and a process for coating paper or board, wherein

-   -   an aqueous polymer dispersion is produced according to the         invention; and     -   a paper coating slip is produced using this polymer dispersion,         at least one pigment and optional further auxiliaries; and the         paper coating slip is applied to at least one surface of paper         or board.

The paper coating slip is preferably applied to uncoated base papers or uncoated board. The amount is generally 1 to 50 g, preferably 5 to 30 g (in terms of solids, i.e. without water or other solvents which are liquid at 21° C., 1 bar) per square meter. The coating can be effected by means of customary application processes, for example by means of a size press, film press, blade coater, air brush, knife coater, curtain coating method or spray coater. Depending on the pigment system, the aqueous dispersions of the water-soluble copolymers can be used in paper coating slips for the base coat and/or for the topcoat.

Paper coating slips according to the invention have good performance properties. They have good running characteristics in paper coating processes and have a high level of binding power. The coated papers and boards have good surface strength, especially a very high wet and dry pick resistance. They are readily printable in the customary printing processes, such as relief printing, intaglio printing, offset printing, digital printing, inkjet printing, flexographic printing, newsprint printing, letterpress printing, sublimation printing, laser printing, electrophotographic printing or a combination of these printing processes.

EXAMPLES

Unless the context indicates otherwise, percentages always signify weight percent. Contents reported relate to the content in an aqueous solution or dispersion. The indication pphm (parts per hundred monomers) denotes the proportion by weight based on 100 parts by weight of monomer.

Where water was used in the context of the examples, demineralized water was used.

Glass Transition Temperature T_(G)

The glass transition temperature is determined in accordance with DIN 53765 using a TA8000 series DSC820 instrument from Mettler-Toledo Int. Inc.

Determination of Gel Content

A polymer film is produced from the dispersion by drying in a silicone mold. Rectangles of approx. 2×1.5 cm side length are punched out from this and weighed. The pieces are then placed in a petri dish with methyl ethyl ketone and left there at room temperature for 48 hours. The uncrosslinked portion of the polymer dissolves in the solvent in the course of this. After this time has elapsed, the polymer pieces are removed from the solvent, dried for one hour at 140° C. and reweighed. The weight loss corresponds to the soluble portion of the polymer and the insoluble portion is the so-called gel. This insoluble weight fraction based on the weight of the dried total polymer is the so-called gel content.

Determination of the 4-PCH Content

The 4-phenylcyclohexene content is determined by means of gas chromatography (direct injection). It is reported in ppm based on the dispersion.

Determination of the Coagulate

The amount of coagulate in the dispersion is based on particles having a diameter of >45 μm. It was determined by filtering the finished dispersion through a sieve with a known pore diameter.

Determination of Thermal Yellowing

The thermal yellowing is determined on coated paper. The coating slip to be tested is applied to one side of a coating base paper using a laboratory coating machine and dried by means of an IR lamp. The weight of the applied coating layer is 10 g/m². The CIE whiteness of the coated side of the resulting paper is determined. Thereafter, it is stored in an air circulation drying cabinet at 120° C. for 48 hours. The CIE whiteness of the stored paper is then measured. The yellowing results from the difference (A) between the two measurements. A value of 0 means no yellowing. The smaller the value, the lower the amount of yellowing.

The following feedstocks were used in the examples:

-   Emulsifier A: sodium lauryl sulfate in the form of a 15% by weight     solution (Disponil SDS from BASF) -   Emulsifier B: ethoxylated sodium lauryl ether sulfate in the form of     a 28% by weight solution (Texapon® NSO P from BASF) -   Complexing agents: EDTA in the form of a 2% by weight solution     (Trilon BX from BASF) -   Seed latex: Polystyrene seed in the form of a 29.7% by weight     dispersion with a particle size of approx. 30 nm (determined by     means of analytical ultracentrifugation) -   Initiator A: 7% by weight solution of sodium peroxodisulfate (NaPS) -   Initiator B: 10% by weight solution of tert-butyl hydroperoxide -   Reducing agent: 13% by weight solution of acetone bisulfite -   Degraded starch: commercial 72% by weight aqueous glucose syrup with     a DE value (dextrose equivalent) of 28

In all examples, the metering of the feeds was effected in a uniform mass flow.

Production of the Emulsion Polymers

The following quantities in pphm (parts per hundred monomer) are based on 100 parts by weight of total monomer.

Example 1 Emulsion Polymerization of Styrene/Butadiene/Acrylic Acid

Initial charge: 47.48 g of styrene (2.11 pphm) 192.86 g of a 7% by weight aqueous solution of itaconic acid (0.6 pphm) 4.5 g of acrylic acid (0.2 pphm) 75.76 g of a 29.7% by weight dispersion of a polystyrene latex with an average particle size of 30 nm (1.0 pphm) 18 g of a 15% by weight solution of sodium lauryl sulfate (emulsifier A) (0.12 pphm) 11.25 g of a 2% by weight solution of EDTA (complexing agent) (0.01 pphm) 86.79 g of a 7% by weight solution of sodium peroxodisulfate (initiator A) (0.27 pphm) Feed 1: 1252.13 g of styrene (55.6 pphm) Feed 2: 90 g of acrylic acid (4.0 pphm) 36 g of a 15% by weight solution of sodium lauryl sulfate (0.24 pphm) 40.18 g of a 28% by weight solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.5 pphm) 37.5 g of 15% by weight sodium hydroxide solution (0.25 pphm) 598 ml of water Feed 3: 842.4 g of butadiene (37.44 pphm) Feed 4: 273.21 g of a 7% by weight solution of sodium peroxodisulfate (initiator A) (0.85 pphm) Feed 5: 231.7 g of a 10% by weight solution of tert-butyl hydroperoxide (initiator B) (1.03 pphm) Feed 6: 66.98 g of acetone bisulfite (0.39 pphm)

The components of the initial charge and 360 ml of water were placed in a 6 l pressure reactor. The components of the initial charge were mixed and heated to 90° C. The polymerization was then started by adding 0.27 pphm of initiator A.

Immediately afterwards, feeds 1, 2 and 4 were begun (time: 0 minutes), feeds 1 and 2 being effected over the course of 4 hours and feed 4 being effected over a period of 4 hours and 15 minutes. The butadiene feed (feed 3) started 30 minutes after the start (time: 30 minutes) of feeds 1, 2 and 4 and was effected over the course of 3.5 hours. Feed 5 started at the same time as the butadiene (time: 30 minutes). For this feed, an amount of 0.8 pphm was metered in over 3 hours. The addition was then interrupted and resumed after 75 minutes. This second metering stage was effected over a period of 2 hours, in parallel with the addition of feed 6. Immediately before the addition of feed 5 was resumed, 15 g of a 15% by weight sodium hydroxide solution (0.1 pphm) were added. Finally, the mixture was cooled down to room temperature and neutralized to a pH of 6-7 with sodium hydroxide solution.

The metering of the various feeds in example 1 can be shown in the following overview:

Time [min] Feed 1 Feed 2 Feed 3 Feed 4 Feed 5 Feed 6 0 Initial charge 90° C. + initiator A + monomers of the initial charge 0 started started started 30 started started 210 stopped (0.8 pphm) 240 stopped stopped stopped 255 stopped 285 Addition of started started sodium hydroxide solution 405 stopped stopped

Example 2 (Comparative Example, without Delayed Addition of the Butadiene)

Example 2 was conducted in analogy to example 1, with the difference that feeds 1 to 4 are started at the same time (all at time 0). Feed 3 ends at the time: 240 min.

Examples 3 and 4

Examples 3 and 4 were conducted in analogy to example 1, the addition of the butadiene being started 60 minutes (example 3) or 90 minutes (example 4) after the start of feeds 1, 2 and 4. Feeds 1 to 3 were ended after 240 min.

Example 5

Example 5 was conducted in analogy to example 1, with the difference that the amount of styrene was increased by 2 pphm (57.6 pphm of styrene) and the amount of butadiene was reduced by 2 pphm (35.44 pphm of butadiene).

Example 6 (Comparative Example, without Delayed Addition)

The emulsion polymerization was conducted as in example 1, with the difference that the metered addition of feed 3 started at the same time as feeds 1, 2 and 4 and ended after 240 minutes. A further difference was that 53.6 pphm of styrene were metered in (reduced by 2 pphm) and the amount of butadiene was increased by 2 pphm (39.44 pphm of butadiene).

Example 7

Example 7 was conducted in analogy to example 1, with styrene being increased by 8 pphm (53.6 pphm of styrene) and the amount of butadiene being reduced by 8 pphm (29.44 pphm of butadiene).

Example 8 Comparative Example to Example 7, without Delayed Addition of the Butadiene

The emulsion polymerization was conducted as in example 1, with the difference that the metered addition of feed 3 started at the same time as feeds 1, 2 and 4 and ended at the same time as feeds 1 and 2 (after 240 min). A further difference was that the amount of styrene was increased by 8 pphm and the amount of butadiene was reduced by 8 pphm.

Example 9 Comparative Example to Example 7

The emulsion polymerization was conducted as in example 1, with the difference that the amount of styrene was increased by 10 pphm and the amount of butadiene was reduced by 10 pphm.

TABLE 1 Overview of the reaction conditions of the various examples and the properties of the polymer dispersions Gel PCH Butadiene Butadiene Styrene T_(G) content content Ex. delay [min] [pphm] [pphm] [° C.] [%] [ppm] 1 30 37.44 55.6 4 91 10 2 n.i. 0 37.44 55.6 9 90 40 3 60 37.44 55.6 −4 90 20 4 90 37.44 55.6 −9 92 15 5 30 35.44 57.6 10 88 15 6 n.i. 0 39.44 53.6 3 93 35 7 30 29.44 63.6 21 88 10 8 n.i. 0 29.44 63.6 26 87 35 9 30 27.44 65.6 26 84 15 n.i.: not according to the invention

All of the dispersions obtained from examples 1-9 had low amounts of coagulate.

Example 10

Initial charge: 93.96 g of a 29.7% by weight dispersion of a polystyrene latex with an average particle size of 30 nm (1.6 pphm) 6.25 g of a 28% by weight solution of ethoxylated sodium lauryl ether sulfate (emulsifier B) (0.1 pphm) 8.75 g of a 2% by weight solution of EDTA (complexing agent) (0.01 pphm) 743.75 g of a 72% by weight aqueous glucose syrup (DE value 28) (30 pphm) 25 g of a 7% by weight solution of sodium peroxodisulfate (initiator A) (0.1 pphm) Feed 1: 997.5 g of styrene (57 pphm) 52.5 g of acrylic acid (3 pphm) Feed 2: 250 g of a 7% by weight aqueous solution of itaconic acid (1.0 pphm) Feed 3: 682.5 g of butadiene (39 pphm) Feed 4: 175 g of a 7% by weight solution of sodium peroxodisulfate (initiator A) (0.7 pphm) Feed 5: 192.5 g of a 10% by weight solution of tert-butyl hydroperoxide (initiator B) (1.1 pphm) Feed 6: 45.42 g of acetone bisulfite (0.34 pphm)

The components of the initial charge were placed in 442.08 g of water in a 6 l pressure reactor. This initial charge was heated to 90° C. 0.1 pphm of initiator A was then added.

Feeds 1, 2, 4 and 5 were then started at the same time, feed 2 being added over a period of 30 minutes and feeds 1 and 4 being added over a period of 2.5 hours. The butadiene feed (feed 3) started 30 minutes after the start of feeds 1, 2, 4 and 5 and was effected over the course of 2 hours (end point after 150 minutes). Feed 5 was stopped after 2.5 hours after the addition of 0.9 pphm. After 3 hours and 30 minutes, 70 g of a 15% by weight sodium hydroxide solution (0.6 pphm) were metered in and then the remaining amount of feed 5 was metered in in parallel with feed 6 over the course of 2 hours. Finally, the mixture was cooled down to room temperature and neutralized to a pH of 6-7 with sodium hydroxide solution.

Example 11 (Comparative Example, without Delayed Addition of the Diene)

The emulsion polymerization was conducted as in example 10, with the difference that feeds 1, 2, 3, 4 and 5 are started at the same time (all at time 0) and are ended at the same time (time 150 min).

Example 12 (According to the Invention)

Example 12 was conducted in analogy to example 10, the addition of the butadiene being started 15 minutes after the start of feeds 1, 2, 4 and 5 and being ended jointly with these.

TABLE 2 Overview of the reaction conditions of examples 10-12 and the properties of the polymer dispersions PCH Butadiene Butadiene Styrene T_(G) Gel content content Ex. delay [min] [pphm] [pphm] [° C.] [%] [ppm] 10 30 39 57 −3 95 <10 11 0 39 57 6 95 25 12 15 39 57 4 95 <10 T_(G) glass transition temperature

All of the dispersions obtained according to examples 1-12 had low amounts of coagulate.

The particle sizes of the polymer particles resulted from the seed control and did not display any significant deviations as a result of the delays in the diene feed.

The advantages of a mode of operation with delay in the diene feed are clearly apparent from tables 1 and 2. The amount of the odor-causing agent 4-PCH is in each case markedly lower than in the examples 2, 6, 8 and 11 not according to the invention. At the same time, a smaller amount of the diene can be used to achieve a desired glass transition temperature (see experiments 1 and 6, 2 and 5, and 8 and 9). When using the process according to the invention, a use amount of butadiene which is reduced by 2 pphm suffices for the production of a polymer with virtually identical glass transition temperature T_(G).

For the same proportion of diene, the gel content of the dispersions is not significantly modified as a result of the delayed feed.

Production of the Paper Coating Slips:

The coating slip was prepared in a stirring apparatus into which the individual components were fed one after the other. The pigments were added in predispersed form (slurry). The other components were added after the pigments, the sequence corresponding to the sequence in the coating slip formulation specified. The final solids content was set by the addition of water.

The coating slip was applied to one side of a coating base paper using a laboratory coating machine and dried by means of an IR lamp. The weight of the applied coating layer was approx. 10 g/m².

Production of a Coating Slip

The reported amounts are based in each case on the solids content.

100 parts by weight of precipitated calcium carbonate (Opacarb A 40 from Specialty Minerals) 9.5 parts by weight of emulsion polymer of the respective example 0.25 part by weight of rheological assistant (Sterocoll FS from BASF SE)

The coating slip was prepared in a stirring apparatus into which the individual components were fed one after the other.

The pigment (precipitated calcium carbonate) was added in predispersed form (slurry). The other components were added in the sequence above. The final solids content was set by the addition of water.

Coating Slip Data:

Solids content 66% by weight Viscosity (Brookfield RVT, spindle 4, 100 rpm) 1000-1400 mPas

TABLE 3 Thermal yellowing of the coated paper d Thermal yellowing Polymer dispersion Δ CIE after 48 h Paper of example Polymer T_(G) [° C.] at 120° C. P1 1 4 30 P2 n.i. 2 9 30 P3 3 −4 31 P4 4 −9 30 P5 5 10 28 P6 n.i. 6 3 33 P7 7 21 25 P8 n.i. 8 26 24 P9 9 26 20 P10 10 −3 31 P11 n.i. 11 6 32 P12 12 4 31 n.i.: not according to the invention

The examples show that papers coated with paper coating slips comprising polymers with the same glass transition temperature exhibit a lower level of thermal yellowing when the polymers have been produced by the process according to the invention.

Example 13 (Not According to the Invention)

Polymerization was carried out in analogy to comparative example 1 of EP 1408059 A1, with the difference that the individual feeds were added in stages with a constant metering rate in each case.

Initial charge: 10.0 g of a 2% by weight solution of EDTA (complexing agent) (0.01 pphm) 59.07 g of a 33% by weight dispersion of a polystyrene latex with an average particle size of 30 nm (0.97 pphm) Feed 1: 40.4 g of acrylic acid (2.02 pphm) 8.0 g of a 20% by weight solution of alkylbenzenesulfonate (Disponil LDBS 20) (0.08 pphm) 649.9 g of water Feed 2: 353.6 g of styrene (17.68 pphm) 7.47 g of tert-dodecyl mercaptan (0.37 pphm) Feed 3: 949.4 g of butadiene (47.47 pphm) Feed 4: 288.6 g of a 7% by weight solution of sodium peroxodisulfate (initiator) (1.01 pphm) 405 g of water Feed 5: 656.6 g of styrene (32.83 pphm) 3.64 g of tert-dodecyl mercaptan (0.18 pphm)

The components of the initial charge and 809 ml of water were placed in a 6 l pressure reactor. The components of the initial charge were mixed and heated to 78° C. Then, feed 4 was started and metered in over a period of 8 hours.

5 minutes after the start of feed 4, feeds 1, 2 and 3 were begun (time: 5 minutes), feed 1 being effected over a period of 6 hours and feed 2 being effected over a total period of 3 hours and feed 3 being effected over a total period of 6 hours.

Feeds 1 and 4 were effected continuously.

Feeds 2 and 3 were metered in the following stages:

Feed 2: 28 g in Feed 3: 181 g in 22 min 45 min 32.5 g in 163.2 g in 22 min 45 min 37 g in 145.4 g in 22 min 45 min 41.5 g in 127.6 g in 22 min 45 min 46 g in 109.8 g in 22 min 45 min 50.5 g in 92 g in 22 min 45 min 55 g in 74.2 g in 22 min 45 min 70.7 g in 56.2 g in 26 min 45 min

After the end of feed 2, feed 5 was started and added over a total of 3 hours. The addition was effected in the following stages:

Feed 5: 56 g in 22 min 62.9 g in 22 min 69.7 g in 22 min 76.6 g in 22 min 83.5 g in 22 min 90.4 g in 22 min 97.3 g in 22 min 123.8 g in 26 min

After the end of feed 4, postpolymerization was performed for one hour and 138.4 g of 10% by weight sodium hydroxide solution were added continuously in the course of this.

Example 14 (According to the Invention)

The example was conducted analogously to example 13, with the difference that feed 3 (butadiene) was started half an hour later (time 35 min) and feeds 2, 3 and 5 were metered in continuously.

The measured values for 4-PCH were:

Inventive example 13: 40 ppm of PCH Inventive example 14: 20 ppm of PCH. 

1. A process for producing an aqueous polymer dispersion by free-radically initiated aqueous emulsion polymerization, which comprises polymerizing, in an aqueous medium, (a) 40 to 75 parts by weight of at least one vinylaromatic compound and (b) 24.9 to 59.9 parts by weight of at least one conjugated aliphatic diene (c) 0.1 to 10 parts by weight of at least one monomer comprising acid groups and (d) 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, the amounts of the monomers (a) to (d) adding up to 100 parts by weight, in a monomer feed process in the presence of a free-radical initiator, with the proviso that the vinylaromatic compound is metered in under polymerization conditions in a continuous mass flow over a period of at least 120 minutes, the metering of the conjugated aliphatic diene is started at a time at which at least 5% and not more than 30% of the vinylaromatic compound has already been metered in in a continuous mass flow and optionally, the polymerization is initiated in an aqueous polymerization mixture comprising up to 5% of the vinylaromatic compound and comprising no aliphatic diene.
 2. The process according to claim 1, wherein the vinylaromatic compound is styrene and/or methylstyrene and the conjugated aliphatic diene is 1,3-butadiene and/or isoprene.
 3. The process according to claim 1, wherein the free-radical initiator is selected from inorganic peroxides and organic peroxides.
 4. The process according to claim 1, wherein a proportion of inorganic peroxide is selected for initiating the polymerization and then the remaining free-radical initiator amount is metered in in a continuous mass flow.
 5. The process according to claim 1, wherein an organic peroxide is metered in starting from the time at which at least 5% and not more than 20% of the vinylaromatic compound has already been metered in.
 6. The process according to claim 1, wherein the conjugated aliphatic diene is metered in in a continuous mass flow over a period of at least 60 minutes.
 7. The process according to claim 1, wherein 15 to 60 parts by weight of a degraded starch are used per 100 parts by weight of the monomers.
 8. The process according to claim 1, wherein no chain transfer agent selected from aliphatic and/or araliphatic halogen compounds, organic thio compounds and substituted thiols is used during the polymerization.
 9. An aqueous polymer dispersion obtainable by free-radically initiated emulsion polymerization according to claim
 1. 10. A method comprising utilizing the aqueous polymer dispersion according to claim 9 as a binder, adhesive, sizing agent for fibers, for the production of coatings or for the production of paper coating slips.
 11. A paper coating slip comprising (i) inorganic pigments and (ii) an aqueous polymer dispersion according to claim
 9. 12. The paper coating slip according to claim 11, wherein the polymers of the aqueous polymer dispersion are used in an amount of 1 to 50 parts by weight based on the total amount of pigments, and wherein the pigments are present in an amount of 80 to 95 parts by weight based on the total solids content and are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminum oxides, aluminum hydroxide, silicates, titanium dioxide, zinc oxide, kaolin, alumina, talc and silicon dioxide, and wherein the paper coating slip additionally comprises at least one auxiliary selected from the group consisting of thickeners, further polymeric binders, co-binders, optical brighteners, fillers, leveling agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.
 13. A paper or board coated with a paper coating slip according to claim
 11. 14. A process for coating paper or board, wherein an aqueous polymer dispersion according to claim 9 is provided; and a paper coating slip is produced using the aqueous polymer dispersion, at least one pigment and optional further auxiliaries; and the paper coating slip is applied to at least one surface of paper or board. 